1. Field of the Invention
The present invention relates to an in-vehicle mount radar device which is mounted in a vehicle or the like, and particularly to an in-vehicle mount radar device which is configured to calculate at least angle information concerning a target.
2. Description of the Related Art
Angle information concerning a target is information representing a direction to an existing target, and it is generally obtained by sweeping a beam in the in-vehicle mount radar device.
A beam is mechanically swept in a mechanical scanning system, for example. According to this mechanical scanning system, a target is detected in an area in which beams are overlapped with each other with respect to two reception antennas, and angle information concerning the target is obtained on the basis of the reception intensity of the beat signal in each area. However, the reception intensity of the beat signal is liable to be affected by various factors. Accordingly, it has been more generally proposed that angle information is obtained by using the phase of the beat signal as compared with the case where angle information is obtained by using the reception intensity of the beat signal. The angle resolution can be enhanced and also the calculation precision of the angle information can be enhanced by using the phase of the beat signal.
A digital beam forming (hereinafter referred to as DBF) processing is known as a technique of achieving angle information by using the phase of the beat signal. In the DBF processing, a transmission pulse which is transmitted from a transmission antenna and reflected from a target is simultaneously received by plural reception antennas, and various beam patterns are formed by digital signal processing by using the reception data. According to a conventional phased array type radar device, an analog phase shifter is connected to each antenna, and it may be estimated that the function of each analog shifter and the function of combining the outputs of the analog shifters in an analog style are implemented by digital signal processing in the DBF processing. In this DBF processing, the reception power intensity and phase of a reception pulse are detected every coming direction of the reception pulse, and the angle information concerning the target can be calculated with high precision by using this phase.
If this DBF processing is adopted, it would be unnecessary to mechanically drive the antenna as in the case of the mechanical scanning system, and thus no driving mechanism would be unnecessary. As a result, an in-vehicle mount radar device which has high resistance to vibration and is designed to be compact and light can be obtained. Furthermore, as compared with the phased array type radar device, no analog shifter is unnecessary and thus the cost of the in-vehicle mount radar device can be more remarkably reduced.
Furthermore, when angle information is obtained by using the phase of reception data, it is known that the angle resolution is generally enhanced more as the aperture diameter of the antenna is larger. However, the in-vehicle mount radar device in which an antenna device must be disposed in a limited space has limited angle resolution because the space is restricted.
For example, according to a radar device shown in FIG. 4 of JP2004-198312A, a reception antenna array is constructed by plural reception antennas which are arranged so as to be spaced from one another at an equal interval of d, and two transmission antennas are respectively provided at both the side portions of the reception antenna array so as to be spaced from both the side of the reception antenna array at an interval of d. A transmission pulse is time-divisionally and alternately transmitted from the two transmission antennas, and the DBF processing is executed by using the reception data obtained by each reception antenna, whereby the antenna aperture diameter is equivalently increased about twice and the angle resolution is enhanced.
However, this publication never discloses that the antenna aperture diameter is further increased, and thus it has been required that the resolution of angle information concerning a target is further enhanced and also the calculation precision of the angle information is further enhanced.